Yield stress and zeta potential of washed and highly spherical oxide dispersions — Critical zeta potential and Hamaker constant

The linear relationship between yield stress and the square of the zeta potential based on the yield stress-DLVO force model, was used to determine the zeta potential at the point of transition from flocculated to dispersed state of a range of oxide dispersions. The critical zeta potential at this point for washed α-Al₂O₃, TiO₂ and ZrO₂ dispersions was of magnitude 40, 49 and 52 mV respectively. For highly spherical silica and alumina dispersions, this value was 23 and 38 mV respectively. The square of the critical zeta potential is proportional to the Hamaker constant of the oxide in water when the van der Waals force is the only attractive force in play. Thus the critical zeta potential data obtained allowed the Hamaker constant ratio between the three oxide dispersions to be determined. This ratio between rutile TiO₂/water and α-Al₂O₃/water was 1.50. In comparison, a similar value of 1.46 was obtained for the ratio calculated from Hamaker constant value determined via Lifshitz theory. The ratio between rutile TiO₂/water and ZrO₂/water is ∼ 0.90. Using the Hamaker constant of rutile TiO₂/water of 61.2 zJ as the standard, the Hamaker constant determined by our method is 41 zJ for α-Al₂O₃, 68 zJ for ZrO₂ and 13.6 zJ for silica.     

Yield stress-zeta potential relationship of oxide dispersions with adsorbed polyacrylate — Steric effect and zeta potential at the flocculated-dispersed transition state

The yield stress-DLVO force relationship is obeyed by α-Al₂O₃ and alumina-coated TiO₂ dispersions with adsorbed polyacrylate only if the yield stress and its corresponding zeta potential data were collected in the positively charged region. In this region, the underlying surface positive charge density of the particles exceeds the negative charge density of the polyacrylate. At this state the adsorbed polyelectrolyte lies flat on the particle surface forming a steric layer of fixed thickness at a given polymer concentration. In the negative charge region, the steric layer thickness is not constant and hence yield stress-DLVO relationship is not obeyed. The (critical) zeta potential at the flocculated-dispersed transition state decreases with increasing polymer concentration. This result reflects a decreasing van der Waals force as the steric layer increases in thickness. A steric layer ensured that the surface or zeta potential is sufficiently low in the flocculated regime for the DLVO theory to remain valid. The ratio of the critical zeta potential square between alumina-coated TiO₂ and α-Al₂O₃ is an indication of their Hamaker constants ratio in water. The effect of alumina coating on the value of this ratio is presented and discussed.     

Turbulent convective heat transfer of nanofluids through a square channel

This paper reports the results of experimental investigation on the heat transfer performance of Al₂O₃/H₂O and TiO₂/H₂O nanofluids through square channel with constant wall temperature boundary condition. The flow regime through channel is turbulent. The nanofluids used in this research are Al₂O₃/H₂O and TiO₂/H₂O with different nanoparticle concentrations. Based on the results of the present investigation, for specific Peclet number, convective heat transfer coefficient and Nusselt number of nanofluids are higher than those of distilled water. The enhancement increases with increasing nanoparticle concentration. The results also reveal that the convective heat transfer coefficient for Al₂O₃/H₂O nanofluid is relatively the same as that of TiO₂/H₂O nanofluid.     

Hydrodesulphurization performance of NiW/TiO2-Al2O3 catalyst for ultra clean diesel

TiO₂-Al₂O₃ binary oxide supports were obtained by sol–gel methods from Tetra-n-butyl-titanate and pseudoboehmite/aluminium chloride resources. The typical physico-chemical properties of NiW/TiO₂-Al₂O₃ catalysts with different TiO₂ loadings and their supports were characterized by means of BET, XRD and UV–vis DRS, etc. The BET results indicated that the specific surface areas of NiW/TiO₂-Al₂O₃ catalysts were as higher as that over pure γ-Al₂O₃ support, and the pore diameters were also large. The XRD and UV–vis DRS analyzing results showed that the Ti-containing supported catalysts existed as anatase TiO₂ species and the incorporation of TiO₂ could adjust the interaction between support and active metal, and impelled the higher reducibility of tungsten. The hydrodesulphurization (HDS) performance of the series catalysts were evaluated with diesel feedstock in a micro-reactor unit, and the HDS results showed that NiW/TiO₂-Al₂O₃ catalysts exhibited higher activities of ultra deep hydrodesulphurization of diesel oil than that of NiW/Al₂O₃ catalyst. The optimal TiO₂ content of NiW/TiO₂-Al₂O₃ catalysts was about 15 m%, and the corresponding HDS efficiency could reach to 100%. The sulphur contents of diesel products over NiW/TiO₂-Al₂O₃ (from pseudoboehmite/AlCl₃) catalysts with suitable TiO₂ content could be less than 15 ppmw, which met the sulphur regulation of Euro IV specification of ultra clean diesel fuel.     

Study on the formation process of Al2O3-TiO2 composite powders

Fine particles of anatase were suspended in solutions of ammonium alum with Al₂O₃/TiO₂ molar ratios from 0.1:1 to 7:1. By spray drying the suspensions and calcining the spray-dried powders, Al₂O₃-TiO₂ composite particles were obtained. The results show that after the spray drying, coatings of ammomium alum are formed on the surface of the anatase particles, leading to composite precursor powders (CCPs) with larger particle sizes. Upon calcining the CCPs, ammomium alum pyrolyzes to amorphous Al₂O₃ and anatase transforms into rutile. Both are mainly responsible for the observed particle size reductions as well as the densification of each composite particle. The in-situ formed α-Al₂O₃ and rutile may have higher reactivities, forming aluminum titanate at 1150 °C, about 130 °C lower than the theoretical temperature for the formation of Al₂TiO₅ by solid reaction. The reaction between α-Al₂O₃ and rutile starts from the interface between the anatase and the alum coating and mainly takes place in the single particles formed by spray drying. The molar ratio of Al₂O₃ to TiO₂ influences the final crystalline phases in the composite powders, but not stoichiometrically.     

Arc Plasma Processing of Pt and Pd Catalysts Supported on γ-Al2O3 Powders

Direct deposition of Pt and Pd nanoparticles onto γ-Al₂O₃ powders was studied by using a pulsed arc plasma process under vacuum to use them as an automotive catalyst. As deposited Pt catalyst exhibited a higher metal dispersion and thus a higher catalytic activity for CO oxidation, compared to the conventional Pt/Al₂O₃ prepared by wet impregnation. In contrast, Pd/Al₂O₃ prepared by the arc plasma method was less active because of its metallic state of Pd with a lower dispersion. A weak interaction between precious metals and γ-Al₂O₃ is not enough for thermal stabilization of as deposited nanoparticles during ageing in a stream of 10% H₂O in air at 900 °C.     

Optimal Ru particle size for selective CO oxidation in H2 over Ru/κ-Al2O3

Ru/κ-Al₂O₃ catalysts with different Ru dispersions were prepared by controlling the pretreatment conditions, and were applied to selective CO oxidation in H₂. The prepared catalysts were characterized by N₂ physisorption, transmission electron microscopy, temperature-programmed oxidation, CO chemisorption, and O₂ chemisorption. The Ru dispersion decreased with increasing reduction and oxidation temperature of Ru/κ-Al₂O₃. The turnover frequency for CO oxidation in H₂ increased as the Ru particle size increased from 2.2 to 3.6 nm, whereas the apparent activation energy decreased as the Ru particle size increased from 2.2 to 3.4 nm for 1% Ru/κ-Al₂O₃. However, larger Ru particles were not always favorable for the selective CO oxidation in H₂ because H₂ oxidation was also promoted by these catalysts. In the case of the 1 wt% Ru/κ-Al₂O₃ catalyst, Ru nanoparticles of approximately 3 nm appeared to be optimal for the selective CO oxidation in H₂ on the basis that they provided the widest temperature window, resulting in complete removal of CO even in the presence of H₂O and CO₂.     

Synthesis of mesoporous TiO2/γ-Al2O3 composite granules with different sol composition and calcination temperature

Mesoporous TiO₂/γ-Al₂O₃ composite granules were prepared by combining sol-gel method and oil-drop method. After calcination at 450 °C, the composite granules showed anatase and γ-Al₂O₃ phases with the specific surface area of 240-310 m²/g. The phase composition and pore structure of the granules can be controlled by calcination temperature and the mixing ratio of boehmite sol and titania sol. The product granules can be used as a photocatalyst or adsorbent in moving or fluidized bed reactors.     

Unusual Activity Enhancement of NO Conversion over Ag/Al2O3 by Using a Mixed NH3/H2 Reductant Under Lean Conditions

Addition of H₂ to a NO/NH₃/O₂/H₂O feed for selective catalytic reduction of nitrogen oxide over Ag/Al₂O₃ catalysts causes an unusual enhancement of activity, e.g., the marginal activity (<10%) of 1 wt% Ag impregnated on γ-Al₂O₃ or mesoporous Al₂O₃ modifications is boosted to nearly 100% over a broad temperature range from 200 to 550°C at a space velocity of 30,000cm³g^?1h^?1). Contrary, silver on SiO₂ or α-Al₂O₃ shows no improvement of activity in the presence of H₂. The effect is tentatively attributed to a higher percentage of intermediary nano-sized Ag clusters on high-surface area Al₂O₃ in the presence of hydrogen. This promotes oxygen activation and hence NO oxidation to reactive intermediate nitrite species. The required dispersion of Ag cannot be stabilized on SiO₂ or α-Al₂O₃.     

Role of Surface Area, Primary Particle Size, and Crystal Phase on Titanium Dioxide Nanoparticle Dispersion Properties

Characterizing nanoparticle dispersions and understanding the effect of parameters that alter dispersion properties are important for both environmental applications and toxicity investigations. The role of particle surface area, primary particle size, and crystal phase on TiO₂ nanoparticle dispersion properties is reported. Hydrodynamic size, zeta potential, and isoelectric point (IEP) of ten laboratory synthesized TiO₂ samples, and one commercial Degussa TiO₂ sample (P25) dispersed in different solutions were characterized. Solution ionic strength and pH affect titania dispersion properties. The effect of monovalent (NaCl) and divalent (MgCl₂) inert electrolytes on dispersion properties was quantified through their contribution to ionic strength. Increasing titania particle surface area resulted in a decrease in solution pH. At fixed pH, increasing the particle surface area enhanced the collision frequency between particles and led to a higher degree of agglomeration. In addition to the synthesis method, TiO₂ isoelectric point was found to be dependent on particle size. As anatase TiO₂ primary particle size increased from 6 nm to 104 nm, its IEP decreased from 6.0 to 3.8 that also results in changes in dispersion zeta potential and hydrodynamic size. In contrast to particle size, TiO₂ nanoparticle IEP was found to be insensitive to particle crystal structure.     

Effect of Support Materials on the Selective Methanation of CO over Ru Catalysts

Selective methanation of CO in the reformate gas (CO/CO₂/H₂/H₂O = 0.175/17.9/70.9/11.1) proceeded over Ru catalysts supported on metal oxides and zeolites. CO was selectively methanated at wide temperature ranges (200–275 °C) over Ru/γ-Al₂O₃, Ru/TiO₂ Ru/H-Y and Ru/H-beta catalysts. Higher Ru contents in Ru/γ-Al₂O₃ improved the selective CO methanation rate.     

Microwave absorption property of polyaniline nanocomposites containing TiO2 and Fe3O4 nanoparticles after FeCl36H2O treatment

It is important to manipulate the synthesis parameters or additives used in order to produce conducting polymer such as polyaniline (PAni) with moderate conductivity, magnetic and dielectric properties that could enhance its microwave absorbing and shielding properties. In this communication, novel PAni/HA/TiO₂/Fe₃O₄ nanomaterials with different Fe₃O₄ contents were prepared by template‐free method by using TiO₂ and Fe₃O₄ nanoparticles as dielectric filler and magnetic filler, respectively. Before addition of ammonium peroxydisulfate (APS) for polymerization, Fe₃O₄ aqueous solution was treated with FeCl₃6H₂O in order to disperse well the Fe₃O₄ in the mixture. The result shows that better dispersion of Fe₃O₄ in the mixture by FeCl₃6H₂O treatment could significantly improve the conductivity of the nanocomposites and also activate the formation of nanorods/tubes. Moreover, PAni/HA/TiO₂/Fe₃O₄ nanocomposites treated with FeCl₃6H₂O show better microwave absorption (99.950–99.999% absorption) compared with PAni/HA/TiO₂/Fe₃O₄ micro/nanocomposites (67.0− 99.4% absorption) without treatment in frequency range of 10–13 GHz. Among the prepared PAni/HA/TiO₂/Fe₃O₄ micro/nanocomposites and nanocomposites, PAni/HA/TiO₂/Fe₃O₄ nanocomposite (treated with FeCl₃6H₂O) with 40% Fe₃O₄ exhibit the best microwave absorption (99.999% absorption at 10 GHz) because of its high conductivity, high heterogeneity and moderate magnetization. POLYM. COMPOS., 2010. © 2010 Society of Plastics Engineers     

Catalytic formation of carbonyl sulfide during warm gas clean-up of simulated coal-derived fuel gas with Pd/γ-Al2O3 sorbents

Coal gasification processes, such as the Integrated Gasification Combined Cycle (IGCC), will increase in importance due to the expanding concern over CO₂ emissions and global climate change. During the development of a Pd/γ-Al₂O₃ sorbent for warm (200 °C) fuel gas cleanup, the catalytic formation of carbonyl sulfide (COS), was observed. This is attributed to a heterogeneous reaction involving fuel gas components (CO/CO₂/H₂/H₂S/H₂O) and Pd/γ-Al₂O₃. The concentration of COS increases 200-fold when exposed to the Pd/γ-Al₂O₃ sorbent. A Langmuir–Hinshelwood reaction mechanism is proposed and a kinetic model is developed based on experimental results. The effect of γ-Al₂O₃, a common catalyst for hydrolysis of COS, and H₂O on the COS concentration is discussed.     

Dispersion of supported platinum catalysts in oxygen I. Experimental evidences of redispersion

The redispersion of platinum on γ-Al₂O₃, SiO₂2 and TiO₂ is experimentally studied by means of hydrogen chemisorption, XRD, TEM, and cyclohexene hydrogenation. The increase in dispersion following treatment in oxygen below 600‡C occurs only for Pt/γ-Al₂O₃. For Pt/TiO₂, only the presence of chloride during oxidation brings about a significant redispersion. For Pt/SiO₂, redispersion does not occur under any condition. Redispersion can occur only in the presence of platinum oxide which can be stabilized by forming a complex with the support. The methods for determining whether or not redispersion will occur for any system and the conditions needed for redispersion are discussed.     

Preparation of meso-macroporous α-alumina using carbon nanotube as the template for the mesopore and their application to the preferential oxidation of CO in H2-rich gases

Meso-macroporous α-Al₂O₃ was successfully prepared by using acid-treated carbon nanotube as mesoporous forming agent and polystyrene foam as the template for the macropore. A series of Ru/meso-macroporous α-Al₂O₃ catalysts were prepared by the incipient wetness impregnation method and applied to the preferential oxidation of CO (CO-PROX) in H₂-rich gases. SEM, N₂ adsorption–desorption, H₂-TPR techniques and TEM were employed to characterize the catalysts. The results indicate that the specific surface area was markedly elevated by introducing the mesopores, which led to the higher dispersion of ruthenium nanoparticles on the surface of α-Al₂O₃. The meso-macroporous α-Al₂O₃ supported ruthenium showed very high activity and selectivity for CO-PROX.     

Gas-phase hydrogenation of maleic anhydride to butyric acid over Cu/TiO2/γ-Al2O3 catalyst promoted by Pd

The promoter effect of palladium on the Cu/TiO₂/γ-Al₂O₃ catalyst was investigated for the gas-phase selective hydrogenation of maleic anhydride to butyric acid at atmospheric pressure. The results show that Pd is added rarely into the Cu/TiO₂/γ-Al₂O₃ catalyst for the hydrogenation of maleic anhydride, the higher selectivity to butyric acid can be obtained. In the absence of Pd (or Cu) in the Cu–Pd/TiO₂/γ-Al₂O₃ catalyst, the selectivity to butyric acid (BA) is nearly zero. Using the Cu–Pd/TiO₂/γ-Al₂O₃ (Pd/Cu=3/100 (atom)) catalyst, 56.2% selectivity to BA and 100% conversion of maleic anhydride were obtained at 280 °C.     

Synthesis of uniform anatase TiO2 microspheres with exposed {001} facets: Without HF utilization and Its photocatalytic selectivity

Uniform anatase TiO₂ microspheres with a high percentage of exposed {001} facets have been successfully synthesized by a one-step hydrothermal strategy in the presence of H₂O₂ and (NH₄)₂CO₃ solution. The as-synthesized anatase TiO₂ microspheres with exposed {001} facets exhibited higher photocatalytic selectivity for degradation of methylene blue and methylene orange mixed solution under UV light irradiation at room temperature than anatase TiO₂ microspheres without {001} facets exposing and P25 TiO₂.     

Synthesis at several length scales of zeolite A membranes using a continuous flow method

Zeolite A membranes were synthesized in the inner side of porous TiO₂ and α-Al₂O₃ tubular supports by a continuous method. The methodology was then used at several scales for preparing membranes on TiO₂ and α-Al₂O₃ supports with lengths of 6, 12 and 25 cm. Formation of an homogeneous zeolite film was confirmed by XRD and SEM in all supports. Single-gas permeation experiments (He, H₂, N₂ and n-C₃H₈) indicated that Knudsen diffusion was the predominant mechanism in both supports. All the synthesized membranes present high flux and moderate selectivity to water in water–ethanol mixtures.     

TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water

TiO₂ photocatalyst loaded on Si₃N₄ (TiO₂/Si₃N₄) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO₂ photocatalysts (TiO₂/SiO₂, TiO₂/Al₂O₃, and TiO₂/SiC) loaded on various types of supports (SiO₂, Al₂O₃, and SiC). The formation of the well-crystallized anatase phase of TiO₂ was observed on the calcined TiO₂/Si₃N₄ photocatalyst, while a small anatase phase of TiO₂ was observed on the TiO₂/SiC photocatalyst and amorphous TiO₂ species was the main component on the TiO₂/SiO₂ and TiO₂/Al₂O₃ photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO₂/Si₃N₄ photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO₂/Si₃N₄ photocatalyst decomposed 2-propanol diluted in water into acetone, CO₂, and H₂O, and finally, acetone was also decomposed into CO₂ and H₂O. The TiO₂/Si₃N₄ photocatalyst showed higher photocatalytic activity than TiO₂ photocatalyst loaded on other supports. The well-crystallized TiO₂ phase deposited on Si₃N₄ and the hydrophobic surface of Si₃N₄ support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions.     

Preparation of spherical α-Al2O3 nanoparticles by microwave hydrothermal synthesis and addition of nano-Al seeds

Due to their special appearance, spherical α-Al₂O₃ nanoparticles play an important role for obtaining high-performance structural and functional ceramics. However, there are still problems such as easily agglomerates to form worm-like structures at high temperatures and difficult availability of spherical nanoparticles. In this study, spherical α-Al₂O₃ nanoparticles with high dispersion were prepared by a combination of a microwave hydrothermal method and an addition of nano-Al particles as seeds. First, spherical amorphous alumina precursors were synthesized by the microwave hydrothermal method at 100°C for 30 min using Al₂(SO₄)₃·18H₂O, Al(NO₃)₃·9H₂O, and urea, as raw materials, and then spherical α-Al₂O₃ nanoparticles with a diameter of about 66 nm were acquired after calcined the precursor at 1050°C for 90 min by adding nano-Al seeds, which reduced the calcination temperature by 50°C and holding time by 30 min compared to that without seeds. Kinetic analysis shows that 5 wt.% nano-Al seeds can reduce the activation energy of crystalline transition of γ-Al₂O₃ to α-Al₂O₃ from 516.51 to 474.37 kJ/mol. Moreover, the microscopic mechanism of nano-Al particles as seeds was investigated. The characterizations of sintering properties show that spherical α-Al₂O₃ nanoparticles facilitate the acquisition of uniform microstructure for resulting ceramic and the fracture modes include both intergranular and transgranular fractures.